Good Tool Rated 5 out of 5 stars
This validates html online. It does that well. The reviewers who want to validate locally in Firefox should go review a local validator add-on and not use this. Right now they are likely reviewing fountain pens claiming they are no good because they are not erasable like pencils.
Rated 1 out of 5 stars
agree with @jaredbeck - assuming (logically) that this is a development tool, most serious developers work locally before uploading to the production site; without the ability to validate locally, it's next to useless.
Not useful for local development Rated 3 out of 5 stars
Remote validation service must have access to target page to be validated. Thus, not useful when developing on localhost.
Can't use it if you can't find it. Rated 3 out of 5 stars
I agree with Gaffer. If you can't find an add-on after it's installed, it's not much use.
If you want it hidden then add an option to do so, after it's been installed and tested.
Rated 3 out of 5 stars
**Reply to Gaffer**
To find it, right click on a blank spot in a toolbar and click "customize". Then you have to drag the Validaty icon into the toolbar for it to show up.
Non-intrusive and well integrated Rated 4 out of 5 stars
This extension has the right approach using a non-intrusive toolbar icon that provides simple graphical feedback, and more in-depth information when errors have occured. It feels like an appropriate form of integration for Firefox.
It would be great to see additional validation as well, particularly CSS validation, as well as having the errors listed in the popup dialog shown from the toolbar icon (in a more terse form) with the option of opening the validator's page in a tab afterwards.